Several approaches have been described that enable the capture of CO2. Some of these approaches allow for controllable release at a later time. One process involves reaction of CO2 with amines, such as ethanolamine, to give an adduct formed by nucleophilic attack of the amine group (RNH2) at the carbon center in CO2. This is typically done in aqueous solution and at relatively high pH, such that the resulting adduct, a carbamic acid derivative (RNHCO2H), is deprotonated to give the anionic carbamate species (RNHCO2−), thereby inhibiting loss of CO2 from solution by volatilization. This deprotonation can later be reversed by increasing the temperature to drive loss of CO2 and thereby recharge the amine for another cycle of capture. Other reaction conditions have been used, for example, in which the amine group is contained within other liquids (e.g., ionic liquids) or in which the amine functionality is immobilized in some way on a solid support, such as silica or carbon. While the reaction of CO2 with amines in these cases is spontaneous and relatively fast, the release of CO2 to regenerate the amine capture agent typically requires the input of a significant amount of energy, usually in the form of thermal energy. Thus, the solution generally must be heated to reverse the CO2 binding and release the CO2 from its trapped state as a carbamic acid or a carbamate. The energy required for this step is related to the heat capacity of the amine solution or amine/support system used for capture. This requirement for large amounts of thermal energy to release the CO2 and to regenerate the compound used to capture CO2 limits the overall efficiency of the round-trip capture/release process.
Another process includes the electrochemical reduction of parent quinones to yield quinone dianions that form adducts including two equivalents of CO2. Oxidation of the quinone dianion-CO2 adduct (an organic carbonate) releases the CO2 and regenerates the quinone. However, it is widely known that quinones react with dioxygen at high rates. Thus, the implementation of quinone capture agents for CO2 from a gas stream, such as flue gas, may not be practical, given that post-combustion gas streams contain high concentrations of O2.
Mineralization has also been discussed as a method to capture CO2. In this case, CO2 is reacted with a variety of minerals (typically oxides or hydroxides) under high temperature and/or high pressure conditions to produce a carbonate-containing or bicarbonate-containing mineral. These processes also consume large amounts of energy and are generally irreversible in a practical sense (i.e., typically reversible only with input of very large amounts of thermal energy).
CO2 capture in ionic liquids (ILs) or in membranes or other support structures that contain ILs has also been described. CO2 can exhibit good solubility in some ILs. For example, CO2 solubility in N-butyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide (BMP-TFSI) can reach as high as 0.1 moles/liter (M) at 300 K and 1 atmosphere of CO2 pressure. However, the reverse of the CO2 capture process is relatively facile, which can lead to easy loss of CO2 from the IL other than at the desired time or location. When amines are attached to ILs as pendent groups, the CO2 that contacts the IL may react with the pendent amine group, thereby forming a carbamic acid or carbamate species. This chemistry can effect the capture of the CO2. However, as noted above, the release step typically requires input of large amounts of thermal energy to drive the binding equilibrium backwards to release CO2 from the carbamic acid or carbamate.